It is expected that the demand for the higher capacity of magnetic recording system has been increased as the development in computer network goes on. The development in the computer network is especially the increase in the variety of uses for large-scale data centers that are compatible with crowd computing. To meet the demand for the higher capacity of hard disks, which are one of the magnetic recording system, may also be a response to the demand for energy-saving. This is because, in the case of hard disks, power consumption per recording capacity can be decreased due to the higher capacity although it has been assumed that the larger-scale data center may cause power shortage.
In hard disks, an aggregation of fine ferromagnetic crystalline grains is treated as one recording bit. Each of the ferromagnetic crystalline grains is a single magnet. Therefore, in order to perform preferable high density recording with less noise, it is necessary to make the ferromagnetic crystalline grains more minute simultaneously with making the area of the recording bit smaller. For example, when the recording density is 10 Gbit/in2, the size of a unit recording bit is approximately 200×300 nm2; when the recording density is 100 Gbit/in2, the size is approximately 75×100 nm2; and when the recording density is 1 Tbit/in2, the size is 20×30 nm2, which is an extremely minute size.
In contrast, it has been known that thermal stability of ferromagnetic characteristic decreases due to the decrease in a grain size in the case when the ferromagnetic crystals are made more minute. In order to stably hold magnetic data (net magnetization and its direction), the product of magneto crystalline anisotropy (Ku) and volume of ferromagnetic crystals (V) have to be a sufficiently large value with respect to a fluctuation factor of heat (KuV/KbT>40-60; herein Kb: Boltzmann constant, T: absolute temperature). Therefore, in order to make the ferromagnetic crystals more minute while erasure of magnetic data due to heat is prevented, it is effective to use a magnetic material with large magnetic anisotropy energy. However, when the magneto crystalline anisotropy energy is increased, coercive force (Hc) is increased as well. Therefore, in order to operate reversal of a magnetization thereof, an even larger writing magnetic field is needed. On the other hand, in a recording head of a magnetic field writing type, which is currently used, the value of saturation magnetic flux density (Bs) of a soft magnetic material used for a magnetic pole reaches 2.4 T, which is close to physical limitation. So, it is difficult to generate further larger writing magnetic field, and a problem that magnetic data cannot be written to a recording medium with high coercive force has occurred.
As described above, the conventional magnetic recording principle, which is a system that operates a state of a recording magnetization (spin) using an external applied magnetic field, faces a substantial issue that is saturation of writing magnetic field intensity of a recording head, in addition to the difficulty of focusing a magnetic field on a local space. It is assumed that the increase in the recording capacity eventually reaches its limit with the conventional method that attempts to realize the increase in the recording capacity by making the ferromagnetic crystals, which is the medium, more minute.
In contrast, a technology has been proposed that writing to a high coercive force recording medium is performed by superimposing additional assist energy such as thermal energy and microwave from external in addition to applying a magnetic field. Thermal-assisted magnetic recording is a technique for performing writing by locally heating particulate recording medium using laser to decrease effective coercive force only during writing. Also, a microwave-assisted magnetic recording is a technology in which a magnetic recording medium is irradiated with high frequency microwave to decrease an effective recording magnetic field due to precession of a recording magnetic moment so that reversal of the recording magnetization is made easy. Both of these technologies are for compensating the lack of a reconrding magnetic field strength from a recording head with respect to a medium with high coercive force, and cannot be an essential solution. Furthermore, in these technologies, since a generation apparatus that generates laser light for heating, an optical waveguide, or a high frequency microwave oscillation apparatus necessary for magnetic resonance is required, there are problems that a thermal design or a configuration of the device is made complicated and further necessary energy consumption during writing is increased.
In order to meet the social needs of the large capacity of recording data and continue to realize a sustainable increase in the magnetic recording capacity, it is necessary to establish a technique that resolves trilemma of magnetic recording that is caused by a necessity of simultaneously realizing the decrease in an area of a recording bit and further minuteness of ferromagnetic grains. Now, several fundamental researches that follow the direction have advanced.
For example, in recent years, a magnetization control method using an electrical means, which is different from magnetization control using only an applied magnetic field, has been getting attention. For example, in WO 2009/133650 (Osaka University); D. Chiba et. al., Nature, 455,515 (2008) (Tohoku University); and Y. Shiota, Nature Materials, 11, 39 (2012) (Osaka Universit), a means that controls magnetic anisotropy of a ferromagnetic metal using an electric field effect has been proposed. Also, in JP Laid-Open Patent Application No. 2007-265512 (Hitachi), a means that controls magnetic characteristics of a magnetic semiconductor using an electrical field effect is proposed. Also, in JP Laid-Open Patent Application No. 2004-342183, JP Laid-Open Patent Application No. 2001-196661 (Sony), and WO 95/22820 (Philips), a means that controls exchange interaction using potential change caused by applying an electric field is proposed, and herein the exchange interaction functions between two ferromagnetic layers.
In contrast, a means that operates magnetic characteristics using electric filed or operates electric characteristics using magnetic field by using a magneto electric effect (hereinafter, to be referred to as ME effect), which is cross correlation of electricity and magnetism, has also been proposed. In the prior art literature, W. Kleemann, Physics, 2, 105 (2009), a magnetic recording head that performs writing using change in magnetic moment caused by the ME effect is proposed. However, the change amount of magnetic moment induced by the ME effect is still limited, and it is difficult to cause sufficiently large change in moment reversal in a magnetic recording medium, etc., to use it for a writing head of a magnetic recording device.
As another means that is different from the above-described means, an attempt to control the strength and direction of exchange-coupling existing between an antiferromagnetic material and a ferromagnetic material using the ME effect is reported. (P. Borisov et. al., Phys. Rev. Lett., 94, 117203 (2005) and H. Xe et. al., Nature Materials, 9, 579 (2010)). FIG. 1A shows a basic structure of Cr2O3, which is an antiferromagnetic oxide; FIG. 1B is a schematic view that shows two antiferromagnetic states (Type 1 and Type 2) that Cr2O3 can take; and FIG. 1C shows respective free energy of the two antiferromagnetic states. Regarding the free energy, this application incorporates references, I.E. Dzyaloshinskill, SOVIET PHYSICS JEPT-USSR, 10, 628 (1960), T. J. Martin et at, IEEE Trans. Magn., MAG-2, 446 (1966). Cr2O3 is a typical ME material that accompanies breaking of space-inversion symmetry & time-reversal symmetry, and spins existing in Cr atoms are lined in the c axis direction. The antiferromagnetic state (Type 1) shown on the left side of the figure and the antiferromagnetic state (Type 2) shown on the right side of the figure generate unidirectional anisotropy (exchange bias) whose direction is 180 degree different from each other in an adjacent ferromagnetic layer. When no external field exists, the energy states (Types 1 and 2) are equivalent. Also, when only either one of an electric field and a magnetic field exists as an external field, energy difference does not occur as well. However, when both an electric field and a magnetic field simultaneously act as external fields, the energy difference between the states occurs in proportion to a product of both of the external fields. For example, the prior art literature N. Wu et. al., Phys. Rev. Lett., 106, 087202 (2011) describes that it is possible to operate an existence probability of either one of the antiferromagnetic states by simultaneously applying an electric field and a magnetic field to an epitaxial thin film of Cr2O3. Also, in the prior art literature, X. Chen, Appl. Phys. Lett., 89, 202508 (2006), and U.S. Pat. No. 7,719,883 B2, a device like a so-called magnetoresistive random-access memory (MRAM) is proposed. The device realizes two different resistive states in a magnetic resistive element such as a spin valve, a TMR element, etc. by switching the two antiferromagnetic states with each other to reverse the direction of an exchange bias acting on an adjacent ferromagnetic layer. However, the previous proposals are made assuming that magnetizations in an entire region of a ferromagnetic layer adjacent to the antiferromagnetic layer part are operated all at once by exchange-coupling as the direction of a magnetization of a pinned layer configuring a MRAM is operated. In other words, a way of selectively operating only magnetizations in an arbitrary region of an antiferromagnetic layer adjacent to an antiferromagnetic layer and a way of making a region to be operated more minute are not considered.
In contrast, in a magnetic recording device such as a hard disk drive, recording of magnetic data is performed by selectively operating the direction of a magnetization in an arbitrary region. In order to further develop the higher density recording that the limitation of the conventional type magnetic recording system such as the above-describe trilemma has been pointed out, a recording technology that can form a recording bit without being influenced by the size of crystalline grains of a ferromagnetic material configuring a magnetic recording medium or the presence of a grain boundary is needed.
FIGS. 2A and 2B are schematic views of a conventional recording method. Here, a method of a perpendicular recording system that applies a magnetic field in the direction perpendicular to a surface of a disk medium is shown. FIG. 2A is an enlarged view of the surface of the disk medium. MF in the figure indicates a magnetization direction, and plus and minus directions are shown by an arrow. The direction corresponds to Z axis direction in the figure. TW denotes a track width of a bit. BP denotes a bit pitch and corresponds to Y axis direction in the figure. Note, a movement direction of the disk medium corresponds to Y axis direction. Also, a situation of an array of bits (recording cells) magnetized in plus and minus directions corresponding to the applied magnetization is shown. In the figure, bits with diagonal lines are magnetized in the plus direction, and bits with blank space mean a magnetization of minus. FIG. 2B is an enlarged view of the circle portion shown by a broken line in FIG. 2A, which is a boundary between plus bits and minus bits. The solid line shown in the center of the figure indicates an actual boundary of an applied magnetic field for writing. It can be said that a state in which magnetization states of bits transit along the boundary is an ideal state. However, the magnetized direction of an actual magnetization is determined by each crystal grain unsteadily existing on the boundary, and therefore crystal grains existing in a manner of covering over the boundary may be partially magnetized in a direction opposite to the original direction. As an example, in the figure, P1 and P2 are originally regions (recording cells) that should be magnetized in the minus direction, but are magnetized in the plus direction. P3 and P4 are originally regions that should be magnetized in the plus direction, but are magnetized in the minus direction. As described above, in the conventional method, the magnetization direction of each grain is changeable, so that detail observation of the magnetization transition region shows that there are some regions of which magnetization directions don't correspond to a general magnetization direction. This width is defined as a magnetic transition width (MTW), and the existence of this width may cause a noise and prevent the high recording density. In order to prevent this phenomenon, a recording method is needed that creates regions of which magnetizations reverse along the boundary of an actual magnetization even in the internals of crystal grains, but don't reverse by each crystal grain.